1. Field of the Invention:
This invention relates to system for encoding a digital signal for transmission, and is particularly directed to a system for protecting a digitized signal, such as digitized audio, from errors which may occur during transmission which can be carried out, for example, by magnetic recording and playback of a PCM audio signal, or by playback of an optical disc.
2. Description of the Prior Art:
In a conventional system for transmitting (or recording) an audio pulse-code-modulated (PCM) signal, an error correcting code is included with the information data. The error correcting code can be a parity code, b-adjacent code, Reed-Solomon code, or other code which will serve to enable correction of errors occurring in the received information data.
In such a system, transmission blocks can be formed of 2m words of the information data together with n error correction words, where m and n are each an integer. When n=1, up to one erroneous word in the block can be corrected, and if greater error protection is required, a larger value of n should be selected.
In the event that the digital signal is transmitted by recording it on tape and thereafter playing it back, dropout can occur causing so-called burst errors. If the block is, for example, formed of (2m+1) words, the occurrence of dropout having a length in excess of one word will cause at least two words in the block to be erroneous, and the erroneous words cannot be corrected.
In order to reduce the prospect that words having such uncorrectable error will occur, it has been proposed to use a word-unit interleaving, or error-spreading technique. In such a technique, upon transmitting (or recording) blocks of (2m+1) words, each of the words thereof is delayed by a different amount so that all of these words will be separated by one or more blocks on the carrier (or record medium). Then, upon receiving (or playing back) this interleaved signal, different respective delay times are provided for each of the 2m+1 words to cancel the delay provided prior to recording to provide error correcting blocks of 2m+1 words. This interleaving technique can be used to disperse, or spread burst errors over several error correcting blocks. If several blocks each have only one erroneous word, all the words can be corrected. In theory, an entire transmission block can be lost to drop out without degrading the error correcting power of this technique.
Unfortunately, upon occasion, errors will occur which cannot be completely corrected. If an uncorrectable error occurs, a masking technique is generally used to provide a synthetic data word to replace an uncorrectable erroneous word. An interpolation technique, which substitutes the average values of data words known to be correct, serves to make any erroneous words inconspicuous. Generally, where the data words represent successive samples of an audio signal, the two data words representing the samples immediately preceding and following the sample represented by an erroneous data word are used to generate an interpolated synthetic data word to mask the erroneous word.
However, when two or more words in succession are erroneous, this masking technique becomes quite difficult, and the uncorrectable erroneous words become more noticeable. In reproduced PCM audio such uncorrectable words are heard as noticeable "clicks."
It has also been previously proposed to design a digital signal transmission system using a plurality of parity or error correction words and doubly-interleaving the information words and parity words to provide additional protection against burst errors. This technique is explained in detail in our Application Ser. No. 218,256, filed Dec. 19, 1980, issued on Oct. 19, 1982 as U.S. Pat. No. 4,355,392.
Nevertheless, despite the success of the foregoing technique, the transmitted signal is still vulnerable to burst errors affecting a number of transmission blocks, so that two successive words in the decoded signal could both contain uncorrectable errors preventing adequate error masking.